1. Field of the Invention
The invention relates to a process for preparing a one-component, H-siloxane-containing addition-crosslinkable silicone resin and to the silicone resin obtained thereby.
2. Background Art
On account of their temperature stability and light stability, which are superior to those of epoxy systems, silicone resins are being used with increasing frequency as encapsulant materials for electronic components. For instance, published specification DE 102 12 119 A1 discloses the high flexibility and softness that characterize such encapsulant materials.
European patent EP 1 249 875 A2 teaches that rubberlike polysilicones are used as encapsulating compounds for producing, for example, standard LEDs. These polysilicones, however, have a surface hardness of 50-90 JISA (Shore D 10), which is too soft for the required surface hardness and scratch resistance. In that patent specification, therefore, a twofold encapsulation is described as well. Over the internal silicone resin encapsulation there is a second encapsulation using, for example, epoxy resins.
Japanese laid-open specification JP 2004-140220 A claims a transparent, addition-crosslinking silicone resin having a hardness of more than 60 Shore D as an encapsulating compound for light-emitting diodes or photo detectors. This kind of silicone resin does exhibit the desired surface properties, similar to the epoxy resins. On account of the low flexibility of such resins, however, the difference in thermal expansion between silicone resin and lead frame causes major problems in the case of the temperature cycle tests the industry requires. Consequently, Japanese laid-open specification JP 2004-140220 A teaches that such silicone resins can be used not as 100% encapsulating compounds, but only in combination with an interlayer between metal lead and encapsulating resin.
Because of these above-recited product properties, high flexibility and softness, or high hardness but low flexibility, encapsulating compounds of this kind can be used only in subsegments of the market, or only with additional manufacturing steps, involving increased cost and complexity.
In European patent EP 1 424 363 A1 and US published application 2005/0212008 A1, Miyoshi describes the preparation of silicone resins for encapsulating compounds by combination of two silicone resin components: component A is a silicone resin with alkenyl groups without H-siloxane (hydrogen-Si groups), while component B is a silicone resin with H-siloxane (hydrogen-Si groups), but without alkenyl groups. Admixed with these two components A and B is an effective catalyst (described as component C).
This silicone resin composition, prepared by mixing together from two silicone resin components, results ultimately in an addition-crosslinked silicone resin having a high hardness (Shore D greater than 60) and a flexural strength of greater than 90 MPa. No details are given of the fracture propensity of the addition-crosslinked silicone resin described, on the basis of the temperature cycle tests the industry requires.
US published application 2002/0161140 A1 likewise describes the preparation of an addition-crosslinked silicone resin from in principle 2 silicone resin components: A, an alkenyl-containing, H-siloxane-free silicone resin, and B, an H-siloxane-containing, alkenyl-free silicone resin. Admixed with these two components A and B is an effective catalyst (described as component C).
A disadvantage is that this composition is composed of two components. This mixture produces addition-crosslinked silicone resins. Owing to the multiplicity of components required, the industrial preparation of such resins gives rise to an extremely large cost and complexity.
The reaction or treatment of H-siloxane-containing silicone resins with aqueous base, aqueous NaOH for example, results in the evolution of hydrogen, a fact which has been known for a long time and is described in “Chemie und Technologie der Silicone”, 2nd Edition, 1968, page 574, section 12.1.2, as well as in references cited therein.